Tried one of these last night and it was pretty tasty. Still not the depth of flavor I would like, but it has only been in the bottle for a little over two weeks. A Russian Imperial Stout probably needs at least two more weeks. Although, last night's example was well carbonated and very clear (not as in see-through, as in no sediment). A week or so in the secondary fermenter seems to have helped a lot. I will be doing this one again, maybe with an extra hop addition.
Had one of these on Wednesday night and it was (compared to the bomber version above) not well carbonated, the flavors were quite muted and a bit astringent. Had another one of these last night and, while not as good as the bomber version, it was ok, pretty well carbonated and no astringent flavors noted.
I think I will toss all of these Grolsch bottles in favor of bombers. They have been sitting around for a long while (measured in years, probably not decades) and I suspect they aren't sealing as well as the capped bombers.
Also noted that all of them had compacted and firm sediment in the bottoms. Didn't even appear to move much during the pour. It also washed out very easily. I am a big fan of this development.